Method of truing gears



Nov. 25, 1930.

A. w. coPLAND E1' AL Re. 17,884

IETHOD OF TRUING GEARS Original Filed April l0. 1922 6 sheets-5115er 2 /q TTORNE YS Nov. 25, 1930. A. w. coPLAND ET AL 17,884

METHOD o'F TRUING GEARs Original FiledfAprl 10, 1922 s shuts-sheet s IIT 1 l INPE/vroRs f ,41 w Cap/and 4m v A 1 nrromvey Noi'. 25, 1930. A. w. coPLAYND ET AL Re- 17,884

METHOD OF TRUING GEARS Original Filed April l0. 1922 6 Sheets-Sheet 4 Defra/1 .Security Trou? Company, Assignee and M. l. Mar/lawson kA Tron/VE Nov. 25, 1930. A. w. COPLAND ET AL l Re- 17,884

METHOD OF' TRUING GEARS Original Filed vApril l0, 1922 6 Sheets-Sheet 5 EEEE'EEE @@@EEI Lw. c and ceased im m ATTORNEYS /Nv /v ORS M Mahewson' f Nov. 25, 1930. I A. w. COPLANDr ET AL. R@ 17,884

METHOD OF TRUING GEARS original Filed April lo 1922 @sheets-sheet e B /NvE/vToRS H. W; Cop/ana' Deceased A b De. fri/' `Secc/r/y Tram Company, Hss'ynee and M. Maf/1e wsoh HTTORNE .tion of Reiesued Nov. 25, 1930 UNITED STATES PATENT-OFFICE ALEXANDER W. Corman-D, DECEASED, BY DETROIT a SECURITY TnUsTcoMrANY, or

DETROIT, MICHIGAN,

ASSIGNEE, MAXWELL I. 'MATHEWSON, OF PROVIDENCE,

RHODE ISLAND,VASSIGNORS TO DETROIT & SECURITY TRUSTl COMP-ANY, OF DETROIT,

MICHIGAN METHDD OETRUING Guns Continuation o! original application Serial No. 551,141, nled April 10, 1922,` andin Great Britain September 14, 1926. Original No. 1,683,867, dated September 11, 1928. Application led February 4,1927, Serial No. 165,867. This application for reissue led August 18, 1930. Serial No. 476;?50.v 1

The invention relates to a methodV of tru'- ing toothed gears, or other bodies having circular series of radiating surfaces, by a grinding or lapping process. Examples of such bodies other than gears are splined shafts, index plates, etc. Since, however, the truing of toothed gears presents all aspects of the invention, the latter will be particularly described with reference thereto. The present application constitu-,tes a continuaour earlier application Serial No. 551,141, filed April 10,1922.

The objects of the invention comprise the correct-ion of defects, first, with respect to the spacing and alignment of the teeth, second, with respect to the concentricity of. the-true pitch circle and the axis of rotation of the gear, third, Wit-h res ect to variations or irregularities in the di erent teeth, and fourth, with respect to the true tooth form.

l.A further object of the invention is to accomplish the foregoing results' by a simple process susceptible of being carried out by means of simple apparatus.

The improved method is based upon the principle thatth'e amount of materialvrevmoved vfrom a surface being ground is 'proportional first, to the area ofthe abrading surface traversing a'unit area. ofthe surface to be ground, second, the pressure of the abradn surface upon a unit area of the survface` to be ground', and-third, the character of the abrading surface. According to this principle, to produce uniform grinding on all portions of a surface contacting withV the abrading surface, there must be uniformity in pressure and area. This is not the case in the ordinary prior processes of lapping gears by rolling the same with intermeshing gears, whether the gears are merely revolved' or are revolved and also axially reciprocated relative to each other.k

According to the present inven-tion there is substituted for the interme'shing spur gear of the prior lnethods a lap in the form of an internal ear with teeth complementary to those of t 1e gear to be trued and-having its circular pitch and the diameter of its pitch circle approximately the same as those of the gear to be trued so as to fit over the latter in steps is uniform telescoping relation. W ith the gear and lap coaxially arranged, they are given a relative reclprocator movementl 4to effect enga ement of their teeth and as the gear and ap start into mutual engagement a torque pres ment. lIn addition thev gea-r and lap are so supported that limited relative movement of the one-1n relation to the other in any direc- .the lapso that different teeth of the gear and lap are brought successively into'engage` '4 tion radial to their axes is permitted with the 4 result that the torsional pressure referred to causes an automatic centeringof the gear in relation to the lap. Suitable abrasive is applled to the mutually engaging surfaces of the lap and gear.

The lapping effected by carrying out these on allsurfaces which are simultaneously in contact. Furthermore, because of the indexing of the gear in relatron to the lap irregularities either in individual tooth form or in the spacing of the teeth are corrected, the efect'of continued lapping with the progressive change in registratron being to continually approach equispacing of the teeth and uniform tooth contour.

lVhile the new method may be carried out with various forms of apparatus, for the sake of specific illustration and'explanation, reference is hadto the accompanying drawings showing by'way of example a form of apparatus Whic'hhas been found satisfactory.

Our method may be carried out with section through cooperating parts.

Fig. 2 is a front elevation, partly in section, f the machine shown in Fig. 1.

F ig. 3 is an enlarged longitudinal section through the indexing head on line 3 3 of Fig. 2.

Fig. 4 is a section through said head in the plane of the variable torque connection on line 4-4 of Fig. 2. y

Fig. 5 is a horizontal section substantially on line 5-5 of Fig. 2. Q

Fig. 6 is a horizontal section on .line 6-6 of Fig. 3. Fig. 7 is a longitudinal section through a construction of annular lap for modifying the toothcontour.

Fig. 8 is a cross scection thereof.

Fig. 9 is a view similar to Fig. 7v of another lap construction for producing a different modifcation of the tooth contour.

Fig. 10 is a diagram illustrating the operation of the machine in the carrying out of the i responding lapping member,

. machine frame.

method. Figs. 11 and 12 are also diagrams showing the manner of altering the torque pressure.

As shown in the drawings the apparatus comprises, first, means for relatively reciprocating the gear to be ground and its cori second, means for intermittently progressivelyT changing the registration of the teeth o f the gear and lap, and third, means for varying the mutual torque pressure of the gear and lap teeth.

Referring in detail to the construction illustrated, A is a suitable frame on which are mounted the center spindles B and C for carryingv an arbor D having the gear E to be ground. ing the lap G. This lap, of suitable material such as cast iron, is in the form of an internal gear having the diameter of.A its pitch circle and its circular pitch approximately the same as those of the spur gear to be ground so as to fit over the latter with the internal teeth of the lap simultaneously engaging the teethof the spur gear. The lap-carrying member F is arrangedy concentric tothe axis of the spindles B and C and mounted by means of an antifriction bearing N upon a crosshead Hwhich `slides upon ways I on the The upper endv'of the lap is additionally supported by an antifriction bearing N between the lap and the crosshead 'H. The member Fis formed with a depending tubular shank F which slidably engages a fixed bearing G carried by the machine frame. The shank F carries splines F which engage straight longitudinal grooves G in the fixed bearing G'. J isla lever for reciprocating the crosshead Hand member 'F actuated by link K connected to a crank L, said crank bein v adjust-able in throw to vary the length o the reciprocation. Obvi'ously the reciprocatiori of the crosshead H effects a corresponding reciprocation of the F is an annular member carry-l forms a support fortlie gear and arboradapted to move laterally somewhat in any direction relative to the lap and its support.

T o change the registration of the gear and the lap on successive strokes, an intermittently operating indexing mechanism is provided. This comprises a dog O on the arbor D, which is engaged by pins or lugs O on a rotatable member O2 secured, as by screws, to ratchet wheel R which in turn is keyed to a sleeve O3. The keyed connection between the ratchet R and the sleeve OS is made secure by the spanner nut O4 which is threaded on the sleeve O3. The latter member is mounted with a lost' motion connection on the' extended hub P of the gear' Q, the lost motion connection being formed by the lug P on the hub P engaging a slotted bearing in the member Oa (see Fig. 6). A light coil spring P2 in thel slotted bearing serves to hold the lugnormally in driving contact with thel member 03. A nut P3 on the end of. sleeveC2 engages hub P and sleeve O3 to hold them rotatably on sleeve C2. The gear Q is connected by a gear train with the drive mechanism for the crank L, said train including a Geneva stop movement, or equivalent device, for converting the continuous rotary motion of the drive into intermittent rotation of the member D.; This, in detail, comprises a gear wheel Q on a shaft Qgconnected by the Geneva stop movement Qa with a shaft Q4, which latter is connected by a l flexible shaftQfv with the shaft Q driven by bevel gear Wheel Q7 from the crank shaft. The arrangement is such that twice during each revolution of the crank shaft, rotary motion is transmitted to the member O sufiicient to advance the teeth ofthe gear wheel being ground into registration with other interdental spaces of the lap. This movement is effected during the portions of the cycle of the crank where the gear wheel is out of engagement with the lap, so that in successive reciprocations or strokes of the crank the gear Wheel has different angular engagements with the lap.

As has been stated, the pressure of the lap against the gear wheel per unit area is one factor determining the grinding action. Where the lap is moved into and out of en gagement with the gear wheel it is evident "that during initial engagement the area of gressively movin contact is small and is progressively increased until the whole of the gear is in contact with the lap. Therefore, if the torque pressure on the arbor D were to remain uniform, there would be a greater grinding action at the ends of the stroke where the con'- tact area is limited than at the center of the stroke where the full area is in engagement. Such a result is avoided by varying the torque pressure during engagement and disen agement and with the construction shown t is is accomplished by transmitting the torque through a' spring and suitably varying the tension of the spring, the specilic devices employed being as follows: 1

The ratch Wheel R which is exchangeably mounted on member Oa and carries the member O2, is engaged by a pawl R carried by a slide R2 mounted in the frame. yieldably actuated by a spring R, which in turn is actuated by a rod R4. S is a member mounted on the crosshead H which is provided with a cam portion S rod R* and actuating the sameto place a tension on the spring R3. The parts are so constructed and arranged that when the gear wheel is out of engagement with the lap, no pressure will be exerted by the cam on the rod R4, and consequently the spring R3 will b e On the other hand, when enters engagement with the also engages the rod R4 prothe latter and thereby increasing the tension of the spring R3. The

at zero tension. the gear wheel lap, the cam S movement imparted to the rod R4 is completed as the gear becomes lfully engaged with the lap so that intermediate the point of initial engagement and the point of full engagement the tensionof the spring Rs has been increased from zero to a predetermined maximuin amount. The tension then remains constant during the further relative reciprocating movement of the lap in relation to the gear until at the opposite end of its stroke it again passes out of engagement, at which time the cam S gradually relieves the tension of the spring to correspond to the diminish'- iiig area of contact. This variation in tension is permitted by the lost-motion connection between the member O3 and the member P formed bv the lug'P engaging the slotted bearing. Thus the pressure of the lap against the gear wheel will be maintained substantially constant per unit area o f surface engagement. It is to be observed in rthis connection that if the pressure is to be kept constant, as last referred to, the speed of the machine, should not be too great else the application of the pressure,v bly the cam V through thespring U, especial y in the case of narrow gears, will be too sudden and not controlled in accordance with the shape of the cam.

It will be noted on reference to Figs. 2, 4 pawl carrying and 5 that a duplicate set of other and turning over the This slide is f for engaging the and actuating arts R2, R3 and RtL are provided at the diametrically opposite side of ratchet wheel R and that a second cam S is provided to actuate them. Thus by changing the pawl R from one carrier R2 to the ratchet wheel R, it is possible to lap both sides of the teeth of a gear without turning the latter over.

To facilitate engagement and disengagement'of the work the center bearing U, together with the sleeve C2, can be raised and lowered by the lever C', having first released the clamp C?. This permits of withdrawing the center whenever the work arbory D is'to be disengaged for the removal of finished work and the engagement of new work to be ground.

I n carrying out the improved method of the kpresent invention by the a paratus just described, or any other suitab e means, one proceeds as follows: The gear Wheel, or other object to be ground, is mounted or in series on Vthe arbor D and the latter is engaged with the center spindles B and C.

corresponding lap G is attached to the annular member F and arranged in vregistration with the gear wheel. The surface of the lap is then coated with abrasive material, after which motion is imparted to the machine to cause 'a 'reciprocation of the lap relative to the gear. At the same time the torque ressure on' the gear is varied as above described, so that during contact with the latter there will be a uniform pressure per unit 'of contact` area. After each reciprocation or stroke and indexing mechanism advances the gear into a different position of registration so that the contacting surfaces areV changed after each stroke. vThe lapping thus effected is continued until the desired truing of the gear is effected.

Assuming that in the gear to be one or more of the teeth are distorted come into Contact ground so as to with the lap in advance of the others, these advanced surfaces will either singly wearing away of the lap is uniformly distributed vover a numberor all of the teeth thereof, whereas the rinding upon the gear continues on the hig iy spots until these are reduced to the level of the other portions. Assuming again that the lap is inaccurate, having certain portions of its surface in advance of the true form, these advanced portions will therefore frst come in contact with the gear teeth and will be .progressively ground away. There will also be a grinding action upon the gear teeth, but due to the progressive change in registration, this grinding is not all on anyone tooth but is distributed over a plurality of teeth. Therefore, not only the gear but also the lap will constantly approach .the true form as the grinding prorelatire to-theV lap or,

manner tendlng to bring mum amount A greater ceeds. In other words, the effect of continuedv grinding is to average the contour and circular pitch oiall of the teeth invboth the gear and the lap and if this average is the' true form, the gear will be rendered true. Since the supporting spindle B is capable of moving laterally somewhat at its upper end, the gear has in a Ineasure a floating support. Hence when mutual torsional pressure is exerted between three or more circumferentially spaced teeth of the lap and a corresponding number of the gear teeth, the gear moves laterally in a manner tending to center itself in other words, in a the pitch Circles of the gear and lap into coincidence. This centering action continues as long as the mutual torsional pressure vis exerted and the result is that all the teeth of the gear are lapped to approximately their true contour with respect to a common gear axis and corresponding pitch circle with the removal of a minifaces.

In-case the average tooth form secured by the above procedure is not the true or desired form, due either to the original cutting or to any other cause, the process as thus far described is incapable of producing the true form. We may, however, by a modification of the method correct an inaccurate average form by'employing a modified lap construction. This modified method forms the subject matter of our application for patent, Serial N o. 551,140, filed April 10, 1922. vIn brief it consists in differentially interrupting the surface ofthe lap so as to produce a grinding effect on one portion of the gear tooth than on another. For instance, if the crown portion of the gear tooth projects beyond the trueinvolute curve desired, this portion may be ground to a greater extent than ,the root portion by cutting notches Win the teeth of the lap, as shown in Fig. 7.' Thus the amount of surfacein the vlap which-rubs over the root of the gear-tooth is less than the amount of surface rubbing over the crown ortion of said tooth which results in removing more material from the crown portion and more closely approximating the true involute curve. The reverse effect may be produced by using a lap such as shown in Fig. 9,

- :he apparatus shown in with teeth cut away' more at the root than at the crown, as indicatedat W', the effect-of this being to grind more atthe root of the gear'tooth than at the crown thereof.

The invention is also applicable to the rinding of helical gears, the modification of Fig. la being applicable for this purpose. Here the fixed bearing G has helical grooves Gr2 instead ofthe straight grooves G of the first construction. The construction is therefore such that during the reciprocationl of the holder the lap of material from the tooth surl so spaced that the or parts with radiating respectively,

- `like to ciprocatingand turning movement and the trosional pressure 1s applied through the gear, 1t 1s obvious that the result'will be the same whetherA these respective relative movements andtorsional pressure of the lap'and gear are effected entirely ythrough one or the other of said members or in ,part through one of them and in'pa'rt through the other.

While the improved method has been described as applied to the grinding of gears, as has previously been pointed out, it is applicable to the grinding of other bodies having' S radiatingsurfaces arranged in uniform circular series. By uniform circular series we mean one in which'the radiating surfaces are circumferential or circular pitch distance between any two of said surfaces is either equal to or a multiple of the circular pitch distance between adjacent radiating surfaces. i

Also, it is obvious that our process is equally applicable to the grinding of both and external gears or other bodies. Thus 1f the gear or body to be ground has its teetn,

surfaces, on an external perimeter, the lap willhave its com-l plementary., oppositely toothedsor surfaced parts on an internal perimeterthereof; and, vice versa, if the gear or other body to be ground has internal teeth or radiating surfaces, the lap will .have external teeth or radiating surfaces. In all cases the pitch circles of the gearand lap will be approximately equal in diameter, and in all cases the teeth of the lap, or its parts with radiating surfaces,'will have contours parallel to the. tooth contours of the gear or body to be lapped. We employ the expressions oppositely toothed, oppositely disposed teeth and oppositely surfaced to indicate the complementary internal-external arrangement of the teeth or radiating surfaces of the gear and lap, respectively, or of the 'lap and gear, as the case may be. While it is preferable'on account of greater easeof manufacture to make the lap teeth fully com- Y plementaryto the teeth of the gear or the n betrued, it will be understoodthat the only complementary relation between the lap and gear teeth which is necesary is .that between .the active 4lapping surfaces, and, 1 since only one lside of a tooth can be active at one time, it would be possible to lap a gear by our method with a lap having teeth with the complementary formation on one side only, it being only necessary after lapping 1 one side of the gear teeth to turn the ear over in order to lap the other sides o its teeth. Also, the term pitch circle when applied to toothed gears, 1s to be understood in its usual technical sense applicable to such ars, whereas when applied to other bodies giving radiating surfaces in circular series it is to be understood as the circle defined by the mean radius of the radiating surfaces.

In our method the effect of continued use of a lap is to true the same inthe same way that the gear is trued, and consequently initial inaccura`cies in the lap are in a measure corrected by the use thereof. This aspect of our invention is of greater importance where equi-spacin ofthe teeth of the gear is the thing chie y sought, .since equi-spacin of the lap teeth is very thoroughly effected by continued use of the lap. If a particular form of gear tooth is to be strictly adhered to we prefer to correspondingl form the lap teeth as accurately as may e initially. Also, it is to be observed that while the use of modified or corrective laps renders high accuracy in theoriginal cutting of the gear unnecessary since the'desired modification of the gear tooth can be effected by the lap, it is obviously desirable if possible to cut the gears with sufficient accuracy so that the use of the modified lap is not required.

While the foreofoing detailed description sets forth the preferred procedure it will be understood that the rocedure described can be varied without eparting from the invention as defined in the appended claims.

What we claim is:

l. The method of grinding gears comprislmgregistering the gear to be ground and a lap having a parallel tooth contour, axially reciprocating the one with respect to the other into and out of mutual engagement, maintaining substantially throughout the mutual overla ping relation of the tWosubstantially uni orm torque pressure per unit area of mutual engagement, and progressively changing the registration between the teeth of the la and those of the gear.

2. The metiiod of grinding gearswhich comprises registering the gear to be ground and a lap having a parallel tooth contour, axially reciprocating one with respect to the other, progressively .rotatively advancing one with respect to the other while out of engagement to change the registration of the teeth, maintainingbetween the gear andlap substantially uniform torque pressure'during said reciprocation while said 'gear and lap are in full contact, and varying the torque pressure while the said gear and lap are entering upon and passing out of full contact to maintain substantially equal pressure per unit area of contact.

3. The method of grinding gears which comprises registering the gear to be ground and a lap, the two being 1n the relation of axially coincident internal and external intermeshing gea-rs, axially reciprocating the one with respect kto the other into and out of mutual engagement, progressively rotatively advancing one with relation to the other while they are out of mutual engagement to change the registration of the teeth, and maintaining throughout the mutually overlapping relation of the two a torque pressure'of t e one on the other which is substantially constant per unit area of mutual engagement.

vet. The method of grinding gears which comprises registering the gear to be ground and a lap, said parts being in the relation of axially coincident intermeshing internal and external gears, axially reciprocating one with respect to the other, rotatively advancing one in relation to the other after each recipro-A cation to change the registration ofthe teeth, maintaining a substantially uniform torque pressure of the one against the other whilel .said parts are in full engagement, and varying the torque pressure at the entering upon and passing out of full engagement to maintain substantially uniform pressure per unit area of engagement.

5. The method of grinding a body, such for example as a toothed ear, having at its perimeter radiating sur aces arrangedin a uniform circular series, said method comprising registering the body with a complementary oppositely surfaced lap having its radiating surfaces parallel in contourto thoseof said body and disposed in a pitch circle of approximately the same diameter as that of said body; axially reciprocatingone in telescoping relation to the other; maintaining a torsional pressure of one upon the other during said reciprocation; and pro ressivelyl changing the regsitration of the body and lap by intermittently angularly advancing one in relation to the other while they are out of mutual engagement.

6. The method of grinding abody, such for example as a toothed gear, having at its perimeter radiating surfaces arranged in a uniform circular series, said method comprising coaxially registering the body with alap having a plurality of radiating surfaces oppositely disposed and of parallel contour to the radiating surfaces 0f the body, the said lap having its radiating surfaces spaced around a pitch circle of approximately the same diameter as the pitch circle ot the said body and' at distances from each other which are equal to or multiples of the circular pitch o the radiating surfaces of said body; axially reciprocating one of said parts in telescoping relation to the other;`maintaining a torsional pressure of one upon the other during their reci rocational engagement; and

rogressively c anging the registration of the Eody and lap by intermittently angularly advancing one in relation to the other while they are out of mutual engagement.

`7.. The niethod of grinding toothed gears which comprises coaxially registering the vgear to be ground with'a lap having oppositely disposed teeth of parallel conteur and approximately the same circular pitch as the gear; axially reciprocating one in telescoping relation to the other; maintaining a torsional :a lap in the relation of axially coincident internal and external intermeshing gears, in axiall reciprocating the one with respect to the ot er in ymutual abrasive contact, and in intermittently progressively rotatively advancingthe one in relation to the other to chan the re 'stration ofthe teeth. y

9.' e met od-of accurately equi-spacing acircularl series' of surface portions of like contour of a. body, such for example as a toothed gear, which comprises coaxially registering the same with a lap having a circular series of complementary oppositely surfaced portions, axially reciprocating the one with respect to the other in mutual abrasive contact, and progressively angularl changing' the re 'stration ofthe body an lap.

10. he method of accurately equi-spacing .a circular series of radiating surfaces of a -fbody,

such for example as a toothed gear, which consists in coaxially registering the same-with a lap having a plurality of surfaces oppositely 'disposed and of parallel contour to said surfaces of the body, the said surfaces of the lap being spaced around a pitch circle of approximately the same diameter Aas the pitch circle of the body and at distances fromv each other which are equal to or multiplesof the circular pitch of the body; axially reciprocating the one with respect to the other in ymutual abrasive contact; and progressively angularl changing tlie'registration of the body an lap l11. The methody of grinding a body, such for example asa toothed gear, having at its perimeter radiating surfaces arranged in a uniform circular series, said method comprising coaxially registering the body with a com lementary oppositely surfaced lap having its radiating surfaces parallel in contour to those of said body and disposed in a pitch circle lof approximately the same diameter as that of said body; creating a mutual torsional ressure between the radiating surfaces o ythe body and lap and thereby moying their pitch circles into coincidence; axially `reciprocatin one in telescoping relation to the other w ile maintaining said torsional pressure; and progressivelyl chan ing the registration of the body and lap by intermit eter as the pitch circle of the body and atv distances from each other which are equal to or multiples of the circular pitch of the body; creating a mutua-ltorsional pressure between the radiating surfaces of the body and the lap and thereby moving their pitch circles into coincidence; axially reciprocating one in telescoping relation to the other while maintaining said torsional pressure ;,and provre'ssively changing the registrationof the body and lap by intermittentlyangularly advancing one in relation to the other vWhile they are out of mutual engagement.`

i3. 'The method of grind'ng helical toothed i gearsl which comprises coaxially registering the gear to be ground with a lap having oppositely disposed teeth of parallel contour and approximately the same circular pitch as thegear; causing a combined axial and rotational movement of the one member relative to the other; maintaining a torsional pressure of one upon the other during said relative movement; and progressively changing the registration' of vthe gear -and lap by intermittently angularly advancing one in relation to the other while they Iare out of mutual engagement.

14. The method of grinding helical toothed gears which comprises coaxially registering the gear to be ground with a lap having oppositely disposed teeth of parallel contour and approximately the same circular pitch as the gear; causing a combined axial and rotational movement of the one member relative tothe other, which movement is positively determined independently of said other member; maintaining a torsional pressure of one upon the other duri-ng said relative movement; and progressively changing the registration of the gear-and lap by intermittently angularly advancing one in relation to the other while they are out of mutual engagement.

15. A method of finishing gear wheels according to which an internal gear wheel is disposed completely to encircle an external gear wheel, the teeth of said gear Wheels being equal in number and complementary in shape, and said gear wheels by relative axial reciprocating motion are then subjected to a series of mutual grinding actions with intermediate circumferential adjustments to engage successively different teeth of said gear wheels, as set forth.

16. A method of finishing gear wheels according to which an internal gear Wheel is disposed completely to encircle an external gear wheel, the teeth of said gear Wheels being equal in number and complementary in shape, torgue is applied to bring corresponding tooth aces of said gear Wheels into operative contact, and Isaid gear Wheels by relative axial reciprocating motion are then subjected to a series of mutual grinding actions with intermediate circumferential adjustments to engage successively different teeth of said gear wheels, as set forth.

17. The method oftruing gearsl which consists in registering a gear and lap, one of which has the form of van'internal gear and the other the form of an external gear, the gear and lap being of the same pitch diameter, having the same number of teeth and formed to fit telescopically one Within the other, opposed tooth faces of the gear and lap being of complemental form and the same in spacing, contour and angularity except for relatively slight inaccuracies, and imparting repeated relative axial reciprocating movements to the gear and lap, one with respect to the other, to cause sliding of lap tooth faces with respect to gear tooth faces in a direction longitudinally of said teeth, While torsionally pressing opposed tooth faces of the gear and lap into Contact during the sliding movements, thereby'causing an abrasive action on the contacting tooth faces which reduces theIv inaccuracies thereof.

18. The method of truing gears which consists in registering a gear and lap, one of which has external teeth and the other of which has internal teeth, with teeth of the lap in spaces between successive teeth of the gear, opposed faces of the lap and gear teeth being of complementa] form and being the same in circumferential spacing, contour and angularity with respect to pitch circles of the same diameter except for relatively slight inaccuracies, and imparting repeated relative axial movements to the gear and lap, one with respect to the other, to cause repeated sliding of lap tooth faces with respect to gear tooth faces in a direction longitudinally of said teeth, while ltorsionally pressing opposed tooth faces of the gear and lap into contact during ing a rasive action on the contacting tooth faces which reduces the inaccuracies thereof.

In witness whereof, we hereto affix our signatures.

MAXWELL l. MATHEWSON. DETROIT & SECURITY TRUST COMPANY, Assignee of Alexander W. Copland, Deceased.

By SELDEN B. BAUME,

Vice President.

the sliding movements, thereby caus- 

